Nickel-Catalyzed Direct Cross-Coupling of Unactivated Aryl Fluorides with Aryl Bromides

A nickel-catalyzed direct cross-coupling of unactivated aryl fluorides with aryl bromides is realized. The one-pot reaction, which avoids the use of preformed and sensitive organometallic reagents, proceeds effectively via C-F bond cleavage at room temperature in THF in the presence of the phosphine ligand and magnesium powder (with or without TMSCl) to produce the desired biaryls in modest to good yields.

Direct C(sp3)-H Polyfluoroarylation: Access to Polyfluoroaryl Amino Acids via Rh-Catalyzed Selective C-F Bond Cleavage

A catalytic selective C-F bond alkylation method for polyfluoroarene with glycinates and derivatives in the presence of a DavePhos-ligated Rh catalyst was developed. This method avoids the preactivation of alkylating reagents and provides an efficient and straightforward route to synthesize a series of polyfluoroaryl amino acids via C(sp³)-H functionalization. This reaction proceeds under mild conditions and exhibits high reactivity and excellent chemoselectivities. Meanwhile, the synthetic potential of this method was demonstrated by gram-scale synthesis, and further transformations proved the application value of the products as well.

Mechanistic Insight into the Ni-Catalyzed Kumada Cross-Coupling: Alkylmagnesium Halide Promotes C-F Bond Activation and Electron-Deficient Metal Center Slows Down β-H Elimination

The Ni-catalyzed Kumada-Tamao-Corriu (KTC) cross-coupling between aryl fluorides and alkyl Grignard reagents has been used to achieve a highly selective Csp²-Csp³ bond construction via the carbon-fluorine (C-F) bond activation. However, the detailed mechanism of this groundbreaking KTC reaction remains unclear. Herein, we perform a series of analyses by density functional theory (DFT) calculations in order to understand the reaction mechanisms for the selective activation of a highly inert C-F bond by Ni catalysts with bidentate phosphorus ligands. An alternative mechanism for Ni/Mg bimetallic cooperation C-F bond cleavage instead of a traditional oxidative addition was proposed. The push-pull interaction in the transition state provided by the Ni center and the Lewis acid of the Mg cation smoothly breaks the C-F bond, supported by the significantly decreased activation energy from 30.9 to 4.6 kcal mol^-1 and principal interacting orbital analysis. Owing to the elevated lowest unoccupied molecular orbital energy level and the electron-deficient metal center caused by the bidentate phosphorus ligand, the β-H elimination could be impeded, increasing the selectivity of KTC cross-coupling. Our DFT results rationally explain the experimental observations, which will be helpful for further development of KTC cross-coupling.

Harnessing Thorpe-Ingold Dialkylation to Access High-Hill-Percentage pH Probes

Sensitivity is an important parameter for a molecular probe. Hill-type pH probes exhibit improved detection sensitivity compared to the traditional pH probes following the Henderson-Hasselbalch equation. Exploiting positive cooperativity, we recently devised a novel molecular scaffold (PHX) to offer such an unconventional Hill-type pH titration profile. We previously confirmed that PHX is not a pure Hill-type probe yet. Only 64% of its absorbance/fluorescence turn-on is the result of a Hill-type pathway. The remaining 36% is from an undesired Henderson-Hasselbalch-type pathway (HH pathway). In this work, the Thorpe-Ingold dialkylation was harnessed to further suppress the percent contribution of the HH pathway down to 16%. We also propose that PHX is a viable molecular model for assessing the efficacy of the steric compressing effect induced by different Thorpe-Ingold dialkylations.

Catalyst-free carbosilylation of alkenes using silyl boronates and organic fluorides via selective C-F bond activation

A regioselective carbosilylation of alkenes has emerged as a powerful strategy to access molecules with functionalized silylated alkanes, by incorporating silyl and carbon groups across an alkene double bond. However, to the best of our knowledge, organic fluorides have never been used in this protocol. Here we disclose the catalyst-free carbosilylation of alkenes using silyl boronates and organic fluorides mediated by tBuOK. The main feature of this transformation is the selective activation of the C-F bond of an organic fluoride by the silyl boronate without undergoing potential side-reactions involving C-O, C-Cl, heteroaryl-CH, and even CF3 groups. Various silylated alkanes with tertiary or quaternary carbon centers that have aromatic, hetero-aromatic, and/or aliphatic groups at the β-position are synthesized in a single step from substituted or non-substituted aryl alkenes. An intramolecular variant of this carbosilylation is also achieved via the reaction of a fluoroarene with a ω-alkenyl side chain and a silyl boronate.

Backbone-Modified C2-Symmetrical Chiral Bisphosphine TMS-QuinoxP*: Asymmetric Borylation of Racemic Allyl Electrophiles

A new C₂-symmetrical P-chirogenic bisphosphine ligand with silyl substituents on the ligand backbone, (R,R)-5,8-TMS-QuinoxP*, has been developed. This ligand showed higher reactivity and enantioselectivity for the direct enantioconvergent borylation of cyclic allyl electrophiles than its parent ligand, (R,R)-QuinoxP* (e.g., for a piperidine-type substrate: 95% ee vs 76% ee). The borylative kinetic resolution of linear allyl electrophiles was also achieved using (R,R)-5,8-TMS-QuinoxP* (up to 90% ee, s = 46.4). An investigation into the role of the silyl groups on the ligand backbone using X-ray crystallography and computational studies displayed interlocking structures between the phosphine and silyl moieties of (R,R)-5,8-TMS-QuinoxP*. The results of DFT calculations revealed that the entropy effect thermodynamically destabilizes the dormant dimer species in the catalytic cycle to improve the reactivity. Furthermore, in the direct enantioconvergent case, detailed calculations indicated a pronounced enantioselective recognition of carbon-carbon double bonds, which is virtually unaffected by the chirality at the allylic position, as a key for the borylation from both enantiomers of racemic allyl electrophiles.

Recent Advances in Transition-metal-catalyzed C-C Bond Formation via C(sp2 )-F Bond Cleavage

The activation of a carbon-fluorine bond is one of the most challenging topics in modern synthetic organic chemistry due to their low reactivity compared to other carbon-halogen bonds. In this review, we present the recent developments since 2015 on cross-coupling reactions that form C-C bonds via cleavage of C(sp² )-F bonds. Not only the conventional activation of C(sp² )-F bonds, but also decarbonylative or carbonyl-retentive cleavage of C(acyl)-F bonds will be introduced. This paper mainly describes new protocols for the formation of C(sp² )-C(sp³ ), C(sp² )-C(sp² ), and C(sp² )-C(sp) bonds via transition-metal-catalyzed cleavage of C(sp² )-F bonds.

Intramolecular Oxidative Coupling between Unactivated Aliphatic C-H and Aryl C-H Bonds

Direct oxidative coupling of different inert C-H bonds is the most straightforward and environmentally benign method to construct C-C bonds. In this paper, we developed a Pd-catalyzed intramolecular oxidative coupling between unactivated aliphatic and aryl C-H bonds. This chemistry showed great potential to build up fused cyclic scaffolds from linear substrates through oxidative couplings. Privileged chromane and tetralin scaffolds were constructed from readily available linear starting materials in the absence of any organohalides and organometallic partners.

Nickel-catalyzed defluorinative alkylation of C(sp2)–F bonds

A nickel-catalyzed defluorinative alkylation of unactivated C(sp²)–F electrophiles using commercially available trialkylaluminum reagents, thus forming the C(sp²)–C(sp³) bonds is reported.

Transition-Metal-Catalyzed Cross-Coupling Reactions of Grignard Reagents

Transition-metal-catalyzed cross-coupling of organo­halides, ethers, sulfides, amines, and alcohols (and derivatives thereof) with Grignard reagents, known as the Kumada–Tamao–Corriu reaction, can be used to prepare important intermediates in the synthesis of numerous­ biologically active compounds. The most frequently used transition metals are nickel, palladium, and iron, but there are several examples for cross-coupling reactions catalyzed by copper, cobalt, manganese, chromium, etc. salts and complexes. The aim of this review is to summarize the most important transition-metal-catalyzed cross-coupling reactions realized in the period 2000 to 2020.

1 Introduction

2 Nickel Catalysis

3 Palladium Catalysis

4 Iron Catalysis

5 Catalysis by Other Transition Metals

5.1 Cobalt Catalysis

5.2 Copper Catalysis

5.3 Manganese Catalysis

5.4 Chromium Catalysis

6 Conclusion

C-F Activation for C(sp2)-C(sp3) Cross-Coupling by a Secondary Phosphine Oxide (SPO)-Nickel Complex

A secondary phosphine oxide (SPO)-nickel catalyst allowed the activation of otherwise inert C-F bonds of unactivated arenes in terms of challenging couplings with primary and secondary alkyl Grignard reagents. The C-F activation is characterized by mild reaction conditions and high levels of branched selectivity. Electron-rich and electron-deficient arenes were suitable electrophiles for this transformation. In addition, this strategy also proved suitable to heterocycles and for the activation of C-O bonds under slightly modified conditions.

Sulfur(IV)-Mediated Unsymmetrical Heterocycle Cross-Couplings

Despite the tremendous utilities of metal-mediated cross-couplings in modern organic chemistry, coupling reactions involving nitrogenous heteroarenes remain a challenging undertaking - coordination of Lewis basic atoms into metal centers often necessitate elevated temperature, high catalyst loading, etc. Herein, we report a sulfur (IV) mediated cross-coupling amendable for the efficient synthesis of heteroaromatic substrates. Addition of heteroaryl nucleophiles to a simple, readily-accessible alkyl sulfinyl (IV) chloride allows formation of a trigonal bipyramidal sulfurane intermediate. Reductive elimination therefrom provides bis-heteroaryl products in a practical and efficient fashion.

Ni-Catalyzed Reductive Liebeskind-Srogl Alkylation of Heterocycles

Herein we present a Ni-catalyzed alkylation of C-SMe with alkyl bromides for the decoration of heterocyclic frameworks. The protocol, reminiscent to the Liebeskind-Srogl coupling, makes use of simple C(sp2)-SMe to be engaged in a reductive coupling. The reaction is suitable for a preponderance of highly valuable heterocyclic motifs. In addition to cyclic bromides, noncyclic alkyl bromides are well accommodated with exquisite levels of retention over isomerization. The protocol is scalable and permits orthogonal couplings in the presence of other functionalization handles.

Nickel-Catalyzed Csp2-Csp3 Bond Formation via C-F Bond Activation

A nickel-catalyzed cross coupling of aryl fluorides via C-F bond activation has been developed. The alkylation method allows selective replacement of aryl fluorides by alkyl groups and enables the synthesis of diverse and otherwise difficult to access scaffolds in good yields.